From: Peter Zijlstra <peterz@infradead.org>
To: "Theodore Ts'o" <tytso@mit.edu>,
Daniel Borkmann <dborkman@redhat.com>,
Andrey Ryabinin <a.ryabinin@samsung.com>,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
Michal Marek <mmarek@suse.cz>,
Sasha Levin <sasha.levin@oracle.com>,
x86@kernel.org, linux-kbuild@vger.kernel.org,
linux-kernel@vger.kernel.org,
Andreas Dilger <adilger.kernel@dilger.ca>,
Dmitry Vyukov <dvyukov@google.com>,
Konstantin Khlebnikov <koct9i@gmail.com>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes
Date: Fri, 24 Oct 2014 12:01:08 +0200 [thread overview]
Message-ID: <20141024100108.GF12706@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20141020141635.GA4499@thunk.org>
On Mon, Oct 20, 2014 at 10:16:35AM -0400, Theodore Ts'o wrote:
> On Mon, Oct 20, 2014 at 04:09:30PM +0200, Daniel Borkmann wrote:
> > >
> > >It's triggering when input_rotate == 0, so UBSan complains about right shift in rol32()
> > >
> > >static inline __u32 rol32(__u32 word, unsigned int shift)
> > >{
> > > return (word << shift) | (word >> (32 - shift));
> > >}
> >
> > So that would be the case when the entropy store's input_rotate calls
> > _mix_pool_bytes() for the very first time ... I don't think it's an
> > issue though.
>
> I'm sure it's not an issue, but it's still true that
>
> return (word << 0) | (word >> 32);
>
> is technically not undefined, and while it would be unfortunate (and
> highly unlikely) if gcc were to say, start nethack, it's technically
> allowed by the C spec. :-)
In fact, n >> 32 == n.
#include <stdio.h>
int main(int argc, char **argv)
{
int i = atoi(argv[1]);
int shift = atoi(argv[2]);
printf("%x\n", i >> shift);
return 0;
}
$ ./shift 5 32
5
On x86 at least the shift ops simply mask out the upper bits and
therefore the 32 == 0.
So you end up OR-ing the same value twice, which is harmless.
So no misbehaviour on the rol32() function.
I think I've ran into this before, in that case I did get fail because I
did indeed expect the 0 and things didn't work out.
next prev parent reply other threads:[~2014-10-24 10:01 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-20 10:54 [RFC] UBSan: run-time undefined behavior sanity checker Andrey Ryabinin
2014-10-20 10:54 ` [RFC PATCH] " Andrey Ryabinin
2014-10-20 19:35 ` Sasha Levin
2014-10-21 8:03 ` Andrey Ryabinin
2014-10-24 8:31 ` y.gribov
2014-10-24 10:36 ` Andrey Ryabinin
2014-10-21 9:47 ` Peter Zijlstra
2014-10-21 10:09 ` Andrey Ryabinin
2014-10-24 10:30 ` Peter Zijlstra
2014-10-21 17:06 ` Randy Dunlap
2014-10-22 9:58 ` Rasmus Villemoes
2014-10-22 11:16 ` Andrey Ryabinin
2014-10-20 11:03 ` drivers: random: Shift out-of-bounds in _mix_pool_bytes Andrey Ryabinin
2014-10-20 12:49 ` Theodore Ts'o
2014-10-20 13:58 ` Andrey Ryabinin
2014-10-20 14:08 ` Theodore Ts'o
2014-10-20 14:09 ` Daniel Borkmann
2014-10-20 14:13 ` Sasha Levin
2014-10-20 14:16 ` Theodore Ts'o
2014-10-20 14:42 ` Andrey Ryabinin
2014-10-24 10:01 ` Peter Zijlstra [this message]
2014-10-24 10:16 ` Andrey Ryabinin
2014-10-24 13:23 ` Sasha Levin
2014-10-24 13:42 ` Peter Zijlstra
2014-10-24 15:04 ` Sasha Levin
2014-10-24 15:10 ` Dmitry Vyukov
2014-10-24 21:05 ` One Thousand Gnomes
2014-10-24 22:23 ` H. Peter Anvin
2014-10-24 22:09 ` Andreas Dilger
2014-10-24 22:22 ` H. Peter Anvin
2014-10-25 0:50 ` Sasha Levin
2014-10-25 20:30 ` One Thousand Gnomes
2014-10-25 20:49 ` Andrey Ryabinin
2014-10-20 11:07 ` kernel: clockevents: shift out-of-bounds Andrey Ryabinin
2014-10-24 10:25 ` Peter Zijlstra
2014-10-20 11:16 ` fs: ext4: mballoc: negative shift exponent Andrey Ryabinin
2014-10-20 11:23 ` jbd2: revoke: negative shift exponent in hash() Andrey Ryabinin
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